The present invention relates to devices intended for continuously and automatically testing the freeness of pulp, i.e. its ability to release a portion of its water content through a screen.
The continuity of the process of testing the freeness of pulp should be understood as the output of data characterizing the freeness said testing being carried out by periodical measurements of the latter during a certain period of time at a frequency ensuring a reliable control of the production process.
More specifically, the present invention is concerned with a device for testing and recording the freeness of pulp, said device being designed for mounting on a pulp line and for carrying out measurements of the freeness of the pulp flowing towards a machine manufacturing, e.g., fiber boards, for the purpose of regulating the process of pulp beating.
The fiber boards are manufactured from vegetable fibers by forming a pulp. The length of fibers and particles is of 0.8 to 1.5 mm, and the thickness thereof is of 0.015 to 0.020 mm. Such a mass is known as a free or a coarsely ground mass whose freeness ranges from 8.degree. to 20.degree. SR. For the purposes of the present invention the term "pulp" designates a mass whose freeness does not exceed 20.degree. SR.
Known in the art are devices for testing the freeness of pulp, presenting reliable information on the freeness (1) within the range of 18.degree.-20.degree. to 60.degree.-65.degree. SR. (L. N. Preobrazhensky et al., Spetsialnye pribory i regulyatory tsellyulozno-bumazhnogo proizvodstva, Moscow, 1972, s. 107-108) and (2) within the range of 60.degree.-65.degree. to 97.degree.-98.degree. SR. (U.S. Pat. No. 4,089,210). At present, the freeness of pulps is determined by means of such laboratory testers as Schopper-Riegler and Defibrator-Sekunda. The Schopper-Reigler tester is insufficiently sensitive to changes in the freeness of pulp within the range of 8.degree. to 25.degree. and beyond 90.degree.. The Defibrator-Sekunda tester in this case possesses higher sensitivity, though both the testers are not suitable for continuous operation directly on the pulp line.
The closest to the present invention from the viewpoint of design features is a device for testing and recording the freeness of pulp (U.S. Pat. No. 4,089,210), comprising a flow receptacle for the pulp whose freeness is to be determined, means for passing the pulp into said receptacle, said means communicating with the latter and designed for mounting on the pulp line, and an immersion type freeness tester. The latter comprises a hollow cylindrical body provided with a bottom screen portion for passing liquid from the pulp. The cavity of said body consists of two chambers communicating therebetween (a filtration chamber and a measurement chamber) from which chambers the filtration one has a greater cross-section, said chamber being provided with the bottom screen portion. There is provided a pneumatic system for immersing said tester into the pulp and removing the same from the latter at certain intervals, and a circuit for recording the result of each measurement carrier out by the tester, said circuit obtaining data on the level of liquid within the tester from the magnitude of the hydrostatic pressure measured by an element sensitive to the level of liquid.
In the above described device the filtrate is first supplied into the internal cavity of the tester for determining the freeness at a pressure determined as a difference between the level of the pulp within the constant level receptacle and that of the pulp at the level of the bottom screen portion of the freeness tester when it is immersed into the constant level receptacle.
As the filtrate is supplied into this tester, the hydrostatic pressure of filtration is dropping, i.e. the filtration process is carried out at a variable pressure which fact adversely affects the result of the measurement of the pulp freeness.
Moreover, at present in the production line, circulating water containing large amount of fine fibers is mainly utilized for beating pulp. In the course of testing the freeness of pulp as above described, due to a great pressure differential of filtration at the initial moment, the fine fibers cover the screen surface and then choke the pores of the free pulp, thereby leading to the wrong evaluation of the freeness thereof, and to an error in the determination of said parameter. The above fact makes it difficult to utilize the above described device for monitoring and regulating the process of beating free pulps.